It has become highly desirable to provide video displays in many formats. There are different modes for different image formats such as panoramic wide screen or movie. There are also different video formats for computer video displays depending upon whether the video display is a VGA (640 pixels in each row.times.480 rows of pixels, where a pixel is a picture element), or a super VGA, or an XGA, Further, there are different television formats from the standard NTSC to as much as 1920 pixels in each row by 1080 rows for High Definition TV (HDTV) format. Further there are flat panel displays and a Digital Mirror Device (DMD), which is a new projection display that utilize reflections from hundreds of thousands of micromirrors, each mounted above its own cell memory.
Within the operation of such a system, it may be desirable to have a panoramic view or wide screen view, or a movie screen view, again requiring some modifications of the number of lines of the video signal from a source to the number of lines in the output signal.
This is achieved by some form of image scaling. In order to achieve these changes in formats, scaling filters have been used. One such known filter is a bilinear interpolation filter, which suffers from the two problems of aliasing (unwanted patterns) and aperture effect, or image softening (blurring). These problems are illustrated in FIG. 1. FIG. 1a illustrates the bilinear filter kernel, FIGS. 1b and 1c illustrate the frequency response.
The aliasing problem can be understood by considering the frequency response of the bilinear filter for an example of a three to four scaling in FIG. 1. Image scaling is basically an image filtering and resampling operation. If X(w) is the frequency domain representation of the original signal, then the resampling operation will create a scaled image whose frequency domain representation contains shifted replicas of X(w), located at the new sampling frequency. These replicas can interact with each other if they are not completely separated from each other, leading to aliasing, as shown in the hatched portion in FIG. 1b.
The second problem is the aperture effect or image softening. This is caused by the fact that interpolation filters are basically low pass filters with high frequency roll-off as illustrated by the frequency response of bilinear interpolation in FIG. 1d.
Both of these problems relate to the frequency content of the signals and can become particularly objectionable with high bandwidth sources.